1. Field of the Invention
The present invention relates to an antenna tuning circuit of a tuner with a band switching device.
2. Description of the Related Art
A prior antenna tuning circuit will be described hereinbelow with reference to FIGS. 4 to 8. FIG. 4 shows a prior antenna tuning circuit of a VHF tuner, FIG. 5 illustrates an equivalent circuit at the reception of a high band, and FIG. 6 illustrates an equivalent circuit at the reception of a low band. Further, FIGS. 7 and 8 are illustrations of transmission characteristics of the antenna tuning circuit at the reception of a high band and at the reception of a low band.
First, in FIG. 4, an input terminal 2 of an antenna tuning circuit 1 is connected through a serial matching coil 3 to a preceding circuit (not shown), for example, an antenna filter (attenuating an intermediate frequency band). This matching coil 3 matches the antenna tuning circuit 1 with the antenna filter or the like being the preceding circuit at the reception of a high band. An output terminal 4 of the antenna tuning circuit 1 is connected through a coupling capacitor 5 to a high-frequency amplifier (not shown). A first low-band coil 6 is connected between the input terminal 2 and the ground, while both end portions of a first high-band coil 9 for reception of a high band are connected through first and second switch diodes 7, 8 to both ends of the first low-band coil 6. That is, the one end of the first high-band coil 6 is grounded by a second switch diode 8 while the other end thereof is connected to the input terminal 2 by the first switch diode 7.
Furthermore, between the input terminal 2 and the other end portion of the first high-band coil 9, in addition to the first switch diode 7, a second low-band coil 10 for reception of a low band is connected in parallel. The other end of the first high-band coil 9 is connected through a second high-band coil 11 to a varactor diode 12 which in turn, is grounded. In addition, another varactor diode 13 is connected to the node between the second high-band coil 11 and the varactor diode 12, and the other end of this varactor diode 13 serves as an output terminal 4 of the antenna tuning circuit 1. Accordingly, the second high-band coil 11 takes a condition put between the other end of the first high-band coil 9 and the output terminal 4. Furthermore, a first capacitor 14 is coupled between the input terminal 2 and the output terminal 4, while a second capacitor 15 is coupled between the other end of the first high-band coil 9 and the output terminal 4.
Incidentally, a capacitor 16 connected in series to the second low-band coil 10, a capacitor 17 put between the first switch diode 7 and the second high-band coil 11 and a capacitor 18 making the first low-band coil 6 grounded are for blocking direct currents. In the antenna tuning circuit 1 thus arranged, in accordance with band switching voltages to be applied to band switching terminals HB, LB, the first and second switch diodes 7, 8 respectively take the conducting or non-conducting condition for the high band reception or low band reception, and a tuning voltage from a tuning voltage terminal TU is applied to the varactor diodes 12, 13, thus being tuned to a give frequency in each band.
FIG. 5 is an illustration of an equivalent circuit of the antenna tuning circuit 1 when the first and second switch diodes 7, 8 in FIG. 4 come into the conducting conditions to allow the high band reception. At the high band reception, the first and second switch diodes 7, 8 assume the conducting conditions, and hence the first low-band coil 6 and the first high-band coil 9 is coupled in parallel to each other, and are shown as a coil 19 connected between the input terminal 2 and the ground in FIG. 5. Further, similarly, the first switch diode 7 gets into the conducting state, and both ends of the second low-band coil 10 are short-circuited, so that the second high-band coil 11 is directly connected to the input terminal 2 as shown in FIG. 5. Moreover, since the first switch diode 7 takes the conducting state, the first and second capacitor 14, 15 in FIG. 4 are coupled in parallel to each other, and are thus shown as one capacitor 20 in FIG. 5. The tuning frequency of the antenna tuning circuit 1 in FIG. 5 mainly depends upon the first high-band coil 19 and second high-band coil 11 coupled in series to each other and the varactor diode 12 connected in parallel to these coils 19, 11.
A capacitor 20 between the input terminal 2 and the output terminal 4 in FIG. 5 is connected in parallel to the second high-band coil 11 and varactor diode 13 connected in series to each other, thereby organizing a parallel resonance circuit whose resonance frequency is set to a substantially intermediate frequency (for example, 650 MHz) of the UHF band, with the result that a signal is attenuated at this frequency. That is, the capacitor 20, the second high-band coil 11 and the varactor diode 13 constitute a trap circuit 21 which attenuates the UHF band at the high band reception, thus preventing the penetration of a UHF band signal at the high band reception. In FIG. 7, a transmission characteristic of the antenna tuning circuit 1 occurring at the high band reception is indicated with a curve A. This curve A arises when the tuning circuit 1 is tuned to 343.25 MHz of a high band, and a reference mark A1 on the curve A represents the tuning frequency of 343.25 MHz and a reference mark A2 denotes the trap frequency of 650 MHz.
On the other hand, at the low band reception, the first and second switch diodes 7, 8 in FIG. 4 come into the non-conducting conditions, and therefore the second low-band coil 10 and the second high-band coil 11 are coupled in series to each other. In this case, the second high-band coil 11 is sufficiently smaller than the second low-band coil 10 and its reactance is ignorable in the case of the low band, and hence the second capacitor 15 in FIG. 4 can be regarded to be coupled in parallel to the varactor diode 13, with the result that the equivalent circuit of FIG. 4 becomes as shown in FIG. 6. In FIG. 6, a variable capacitor 22 is due to the parallel connection of the second capacitor 15 and the varactor diode 13. Further, the first capacitor 14 is connected in parallel to the second low-band coil 10 and variable capacitor 22 connected in series to each other, and its parallel resonance frequency is set to a vicinity of an image frequency to the low band frequency. In FIG. 8, a curve B shows a transmission characteristic taken when the antenna tuning circuit 1 in FIG. 4 is tuned to 126.25 MHz of the low band, wherein reference mark B1 represents the tuning frequency of 126.25 MHz while B2 represents the trap frequency of 240 MHz set to a vicinity of the image frequency (European specification).
At the high band reception, for example, when receiving 343.25 MHz, the image frequency becomes 421.05 MHz, while as obvious from FIG. 7 the transmission level (A3 on the curve A) at this frequency in the above-described prior antenna tuning circuit 1 only attenuates by approximately 17 dB (X1 in FIG. 7) with respect to the tuning frequency A1. For this reason, it is disturbed from the image frequency at the high band reception. One possible solution to this problem is to lower the prior trap frequency (A2 on the curve A in FIG. 7) set to the UHF band. However, in this case, the attenuation of the UHF band does not occur and, hence, the disturbance from the UHF band frequency takes place, which makes difficult the resolution of the problems.
In addition, in the FIG. 6 equivalent circuit at the low band reception, the trap circuit 23 shows capacitative at its both ends, that is, between the input terminal 2 and the output terminal 4 in the tuning circuit, at frequencies higher than the trap frequency, and hence, has a serial resonance frequency with the matching coil 3. This frequency appears in a lower band of the UHF band, i.e., 400 MHz, so that a peak point (B3 on the curve B in FIG. 8) arises as the so-called bounce. As indicated by Y1 in FIG. 8, this peak point B3 only attenuates by 3 dB with respect to the low band tuning frequency B1, so that the low band is disturbed from this frequency.